Amid a flurry of Republican calls for a special prosecutor to investigate the Internal Revenue Service’s alleged targeting of conservative groups, the Justice Department is set to defend its own probe of the matter as exhaustive and sufficient during a House hearing Thursday on the department’s response to the scandal.

A top Justice Department official will tell the hearing that the department is now looking at how some of former senior IRS official Lois Lerner’s emails disappeared -- a new area of inquiry, Attorney General Eric Holder told ABC News in an exclusive interview Friday.

“We've expanded the scope of the IRS investigation to deal with these lost emails,” Holder told ABC News’ Pierre Thomas.

Specifically, the Justice Department and FBI are looking into “the circumstances of the lost emails,” Deputy Attorney General Jim Cole will tell a House Oversight and Government Reform subcommittee Thursday, according to excerpts of his prepared remarks provided to ABC News.

Last month, the IRS revealed it lost two years’ worth of Lerner’s emails when her hard drive crashed in 2011. The vanished emails spanned January 2009 to April 2011, when the alleged IRS targeting of conservative groups “was occurring regularly,” as two Republican lawmakers recently put it.

Lerner’s attorney, William W. Taylor III, has dismissed the whole IRS controversy as “baseless" and not a "conspiracy.” But, in discussing the Justice Department probe, FBI Director James Comey has said there is “a reasonable basis to believe that crimes may have been committed.”

In his interview with ABC News on Friday, Holder said prosecutors and FBI agents -- not political appointees -- are “doing a good, professional job” investigating whether conservative groups seeking tax-exempt status from the IRS received undue scrutiny.

“I've been briefed on the progress that they are making,” Holder said. “And I'm satisfied.”

The Republican-led House passed a resolution in May calling on Holder to appoint a special prosecutor to investigate the matter, and many Republicans in the Senate have echoed that call.

In January, the House Republicans leading the congressional probe into the IRS scandal sent a letter to Holder, questioning the “integrity of the DOJ/FBI investigation.”

Specifically, the letter from Rep. Darrell Issa, R-California, and Rep. Jim Jordan, R-Ohio, noted that a federal prosecutor who has donated nearly $7,000 to President Obama’s political campaigns and other Democratic causes was coordinating the Justice Department’s investigation.

That “has created a startling conflict of interest ... and has compromised the administration’s investigation of the IRS,” said the letter, which also cited “indications that the [FBI] was not taking the investigation seriously.”

Three weeks later, during a Senate Judiciary Committee hearing, Sen. Lindsey Graham, R-South Carolina, told Holder that appointing a special prosecutor “is a no-brainer” because it would avoid even the appearance of bias.

But Holder noted that even “career people” -- government workers not politically-appointed -- have the constitutional right to donate to political causes, and he has rebuffed efforts to appoint a special prosecutor.

“The men and women of the Justice Department have from time immemorial put aside whatever their political leanings are and conducted investigations in a way that rely only on the facts and the law,” he said. “I don't think there's any basis to believe that anybody who was involved in this [IRS] investigation would conduct themselves in a way that is inappropriate or would be shaded by their political activity.”

The House subcommittee’s hearing Thursday is focused solely on the Justice Department’s response to the IRS scandal, and Cole -- the sole witness testifying -- will continue to defend what Republicans say is the department’s “refus[al] to address the bipartisan concerns of the House by appointing an independent prosecutor.”

Cole, though, will not offer details about what the Justice Department’s investigation has found so far, according to the excerpts of his opening remarks.

“While I understand that you are interested in learning about the results of the investigation, in order to protect the integrity and independence of this investigation, we cannot disclose non-public information about the investigation while it remains pending,” he is expected to tell the House panel. “This is consistent with the longstanding department policy, across both Democratic and Republican administrations, which is intended to protect the effectiveness and independence of the criminal justice process, as well as privacy interests of third parties.

“While I know you are frustrated by the fact that I cannot at this time disclose any specifics about the investigation,” he is expected to add, “I do pledge to you that when our investigation is completed, we will provide Congress with detailed information about the facts that we uncovered and the conclusions we reached in this matter.”

In May, the Republican-led House voted to find Lerner in contempt of Congress for refusing to answer lawmakers’ questions about the IRS scandal. Lerner headed the IRS division that processes applications for tax-exempt status, but she has since retired.

ABC News’ Julia Noel and The Associated Press contributed to this report.